It is well known that marketeers and security services often use data-mining techniques to extract private information by collecting huge amounts of data from web visits, emails, purchases, and more.

Recently Google security team researchers released an alerting announcement warning that by becoming involved in social networks online, users are building up their own piles of revealing data. And as more websites gain social features, even the things users strive to keep private will not necessarily stay that way.

An example: by combining public information on in which two different social networks are linked or shared could reveal information of your contacts surfing habits in another CMS or social network site, even if the profiles are kept relatively anonymous and are not linked directly in any way.

This approach is often called "merging social graphs" by the researchers. In fact, it has already been used to identify some users of the DVD rental site Netflix, from a supposedly anonymous dataset released by the company. The identities were revealed by combining the Netflix data with user activity on movie database site IMDb.

The Google security team's proposed solution is a kind of privacy warning system. When you sign up for a new online service, it would take a look on the internet and let you know if there's a risk that the new information you are uploading could be used to make connections about you.

In 2007 computer scientists at Palo Alto Research Center, California, and the University of Waterloo, Ontario, built a similar warning system. It calculates whether data about to be released - for example medical records sent to insurers - could be combined with publicly available information - for example wikipedia articles on health conditions - to reveal diagnoses purposefully removed from the original data.